The information carrying capacity of an optical communication system can be increased by wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). Unless otherwise stated, the term wavelength division multiplexing will be used in this disclosure to encompass both wavelength division multiplexing and dense wavelength division multiplexing.
In wavelength division multiplexing, multiple optical signals, each having a different wavelength, are carried by a single optical fiber. Each optical signal is modulated with a different information signal. Thus, a single optical fiber can carry n information signals, where n is the number of optical signals carried by the optical fiber.
The optical signals are multiplexed before being fed into the optical fiber. At the output of the optical fiber, the optical signals are de-multiplexed so that the information signal carried by each optical signal can be recovered. Alternatively, the optical signals, or a sub-set of them, may be transferred to another optical fiber for onward transmission.
Recently, tunable lasers whose emission wavelengths are capable of being tuned over a wavelength range that spans multiple channels of a wavelength-division multiplexing system have been proposed. Using a tunable laser to generate an optical signal for transmission via an optical communication system that employs WDM further increases the practical information carrying capacity of the optical communication system. For example, the wavelength of the light generated by the tunable laser can be set to the wavelength of a currently-unused channel of the optical communication system. This allows the unused channel to transmit an information signal. Later, when an information signal having a higher priority is scheduled to use the channel, the wavelength of the tunable laser can be changed to that of another channel that is currently unused. Transmission of the information signal can then continue via the other channel.
Incorporating a tunable laser in one or more of the optical signal sources incorporated in an optical transmitter connected to the optical communication system enables such optical signal sources to transmit information signals via channels that would otherwise be unused or under-used.
However, tuning a tunable laser between a source channel and a destination channel causes the problem of a momentary high level of noise in each channel that lies between the source channel and the destination channel. The high level of noise occurs in a given channel as the wavelength of the light generated by the tunable laser sweeps through the wavelength range of the channel. Such noise is usually unacceptable as it disturbs the integrity of the information signal transmitted via the channel. In addition, the optical communication system typically includes amplifying and other optical elements that can be caused to malfunction by the signal from the tunable laser as the laser is tuned.
Thus, what is needed is a multi-channel optical transmitter incorporating a tunable optical signal source in which tuning the tunable optical signal source does not cause high levels of noise in the channels over which the wavelength of the light generated by the tunable optical signal source is tuned.